i) Field of the Invention
The present invention relates to a polymer having an amino acid residue or an amino acid ester residue as a pendant group in at least a part of a monomer unit, and a process for preparing it. More specifically, the present invention relates to a process for preparing a polymer by reacting polysuccinimide with an amino acid ester.
That is to say, the present invention relates to a polymer which can be obtained by reacting polysuccinimide with an amino acid ester in the presence or absence of a basic catalyst and which is not irritant to organisms (e.g., eyes and skin) and which has an amino acid residue or an amino acid ester residue as a pendant group in at least a part of a monomer unit, and a process for preparing it.
The polymer of the present invention is characterized by having no irritation to organisms (e.g., eyes and skin), and for example, the polymer is useful in the industrial fields of medical supplies, health, beauty and sanitary aids, quasi-pharmaceuticals (in Japanese: "IYAKU-BUGAIHIN" defined in the Drugs, Cosmetics and Medical Instruments Act of Japan), and the like. Concretely, for example, the polymer is useful in the industrial fields of carriers for drugs, cosmetics, perfumes, surface active agents, food additives (thickeners, stabilizers, moisturizers, noodle modifiers, coagulants, pH adjustors, bacteriostats and the like).
ii) Description of the Prior Art
Aspartamides obtained by ring opening reaction of polysuccinimide with various amines have been known ever since a long time ago.
For example, in Journal of Medical Chemistry, Neri et al., Vol. 16, No. 8, p. 893-897, (1973), there has been disclosed a technique in which the rings of polysuccinimide are opened by reaction with ethanolamine to produce 2-hydroxyethylaspartamide. This product is considered to be useful as a plasma expander.
For example, U.S. Pat. No. 4,363,797 (Jacquet et al.) has disclosed a technique in which the rings of polysuccinimide are opened by reaction with cysteamine or taurine to produce polyaspartamide. This product is considered to be useful as a cosmetic composition.
However, the present inventors observed that the polyaspartamide obtained by ring opening reaction of the polysuccinimide with cysteamine is an irritant when they carried out a rabbit eye mucous membrane irritation test (described hereinafter).
Therefore, it has been apparent that the polyaspartamide, which is obtained by ring opening reaction of the polysuccinimide with cysteamine according to the prior art, does not always provide a polymer having no irritation to organisms.
With regard to this problem, it is not clearly definite what substance causes the irritation to the rabbit eye mucous membrane. However, the present inventors have presumed that the amine such as cysteamine used for the ring opening of the polysuccinimide is concerned with the irritation in any manner.
Thus, in view of the above-mentioned problem of the prior art, the present inventors have intensively conducted investigations based on a conception to use a compound, which is safe to organisms and environments, for the ring opening reaction of the polysuccinimide. As a result, it has been found that a polymer supposed to be substantially free from the irritation to organisms and to have an extremely high safety to organisms can be obtained by using an .alpha.-amino acid, which is one having a good adaptability and an extremely high safety to organisms, for the ring opening reaction of the polysuccinimide. In consequence, the present invention has been completed.
A technique whereby that the rings of the polysuccinimide are opened by using an .alpha.-amino acid, which is one having a good adaptability and an extremely high safety to organisms, has not been publicly known at all prior to the filing of the present application.
The reason why a technique whereby the rings of the polysuccinimide are opened by using an .alpha.-amino acid has not been publicly known at all prior to the filing of the present application, though it has been known that the .alpha.-amino acid has a good adaptability and an extremely high safety to organisms, is probably based on the fact that technical problems (e.g. the low solubility of .alpha.-amino acid in an organic solvent and the very low reactivity of the .alpha.-amino group) have not been solved yet.
Very few techniques whereby the rings of the polysuccinimide are opened by using an amino acid other than the .alpha.-amino acid have been publicly known before the filing of the present application. This reason is also probably based on technical problems regarding the solubility in an organic solvent and the low reactivity of an amino acid other than the .alpha.-amino acid.
With regard to the technique about using the amino acid, only Die Angewandte Makromolekulare Chemie-Applied Macromolecular Chemistry and Physics [Machado et al., Vol. 195, p. 35-56 (1992)] disclosed a technique whereby the rings of the polysuccinimide are opened by using .GAMMA.-aminobutyric acid (piperidic acid) which is .GAMMA.-amino acid or glycylglycine which is oligopeptide (dipeptide).
In the Machado et al technique, however, even if four equivalents of glycylglycine is charged to the reaction per one equivalent of the monomer unit of the polysuccinimide in the presence of a strongly basic catalyst, only 0.5 equivalent of glycylglycine actually takes part in the reaction. Consequently, the problem regarding low reactivity has not been solved yet.